Water-based central heating systems were originally developed 
in the late 1800s44. During that time, and for several decades to follow, there were no electrically powered circulators available. The only “propulsion effect” available to move water through systems was the differential pressure created by the simultaneous presence of hot and cool water. Hot water in the boiler is slightly lighter than cooler water in the return side piping.
This creates a slight weight imbalance that causes hot water to rise while cool water descends, and thus circulation occurs within the system.
These early hot water systems were often referred to as “gravity” hot water systems. An example of the piping used in these early hydronic systems is shown in opposite figure.
Notice that the piping is either vertical or slightly sloped.
This slope encourages buoyancy driven flow and helps prevent air pockets from forming within the piping. Such pockets could block flow.
These early hydronic systems were open to the atmosphere at the overflow pipe. As such, the pressure within the boiler and piping were limited. In this respect, they were safer than pressurized steam systems of that vintage. However, due to evaporation, the water level within the system would slowly drop. Water had to be periodically added to maintain proper operation.
Although they took great advantage of natural principles to create circulation, these early systems were very limited in how they could be applied relative to modern hydronic systems.
The first-generation hydronic radiant panel systems used “grids” and “coils” of steel or wrought iron piping embedded in concrete slabs. After World War II, soft temper copper tubing became the preferred material for such systems due to its availability in smaller sizes and easier bending characteristics. During the later 1940s and early 1950s, radiant floor and ceiling heating systems using copper tubing became increasingly popular.
Although these radiant panels delivered exceptional comfort for their time, and some are still in use today, others failed prematurely due to chemical reactions between copper tubing and certain materials used in concrete. Repetitive mechanical stresses due to the heating and cooling also caused some failures. Such issues eventually tarnished the reputation of hydronic radiant panel heating. By the 1970s, new installations of copper-based radiant panel heating systems were almost non-existent.
Most residential hydronic systems of the 1960s through 1970s shifted to cast iron baseboard or copper fin-tube baseboard. Pressure-rated boilers with heat exchangers made of cast iron sections or steel fire tubes and fuelled by natural gas and fuel oil became common. Hydronic systems of this era were relatively simple, typically having 1 to 3 zones and basic electromechanical controls.
These devices allowed the boiler to provide domestic hot water as well as space heating. They also required the boiler to remain at an elevated temperature of at least 140ºF for the entire year. Although acceptable at the time, this method of domestic water heating is now viewed as very inefficient.
In the early 1980s, crosslinked polyethylene tubing (e.g., PEX) made its way to North America after several years of successful use in Europe. PEX, and other polymer-based tubing, revolutionized the installation of hydronic radiant panel heating, providing fast installation and a long, reliable life. The availability of PEX tubing was the spark that rekindled interest in hydronic radiant panel heating in North America during the 1980s.
In the early 1980s, crosslinked polyethylene tubing (e.g., PEX) made its way to North America after several years of successful use in Europe. PEX, and other polymer-based tubing, revolutionized the installation of hydronic radiant panel heating, providing fast installation and a long, reliable life. The availability of PEX tubing was the spark that rekindled interest in hydronic radiant panel heating in North America during the 1980s.
MODERN HYDRONICS:
Today, much of the technology available for hydronic heating is a quantum leap above that available in the mid-1900s. Examples include the following:
- Compact wall-hung condensing boilers, such as the one shown in opposite Figure, when properly applied, can operate with thermal efficiencies in range of 95%.
- Renewable energy heat sources, such as solar thermal collectors, heat pumps and biomass boilers, can be integrated into hydronic systems.
- Multiple water temperatures within the system can be precisely controlled using both electronic and thermostatic devices.
- A wide assortment of heat emitters is available to match heating loads and aesthetic preferences.
- Systems can heat buildings as well as supply domestic hot water, melt snow on outdoor surfaces and warm swimming pools.
- Heat sources, mixing devices and circulators can be
intelligently controlled using microprocessors. - Systems can be designed that use a fraction of the fuel and electrical
energy required by earlier systems, yet provide superior comfort. - System operation can be monitored and altered, if necessary, using a laptop, tablet or smart phone from any location with Internet access.
energy required by earlier systems, yet provide superior comfort.